1. Field of the Invention
This invention relates to an osteosynthesis plate or a comparable implant plus a ball socket for such an implant.
2. The Prior Art
A problem frequently encountered in orthopedic surgery is that bone fragments which have been separated due to a fracture must be joined together for the purpose of support and/or healing. Osteosynthesis plates and comparable orthopedic implants have proven suitable for this purpose. They have specially designed receiving bores to receive bone screws. The implants are intended in particular for fixation of unstable fractures of large or small tubular bones as well as compact bones such as vertebral bones, wrist bones or ankle bones.
Numerous implants of the generic type in question have been proposed; they can be divided into roughly three groups:
First, traditional osteosynthesis plates with a spherical or conical screw seat or a spherical sliding seat for dynamic compression are known. These are simple plates made of a biocompatible metal into which bone screws can be screwed through predetermined holes in the bones. The holes in the plates are shaped in different forms and have countersunk bores in the form of spherical or conical round bores or spherical sliding holes for centering the screw heads. When the screw is tightened in the bone, the plate is tightened and secured against the bone.
Such plate-screw joints allow almost exclusively the transfer of tensile forces in the direction of the screw axis as well as transverse forces. However, there is no angular stability with this type of plate-screw joint. Therefore, the elementary prerequisite for the use of traditional osteosynthesis plates is an adequate gripping strength of the screw thread in the bone, which makes it possible to secure the plate tightly enough against the bone. In particular in osteoporosis and in the joint area of tubular bones, however, it is often impossible to achieve adequate holding forces of the screw thread in the bone. Then the screws may become loosened and the bone fragments may become tilted, resulting in failure of the implant.
In addition, there are known monoaxially stable plate-screw joints, where bone screws are screwed into the bone through bores in the osteosynthesis plate at a certain angle that is predetermined by the structure. These bores are designed so that the screws are secured in the osteosynthesis plate at this predetermined angle. This makes it possible to hold even compact bone fragments at a fixed angle in relation to the bone plate in the case of periarticular fractures without having to secure the plate against the bone. The periosteum is advantageously protected in this way. Such implants are known from U.S. Pat. 5,053,036, U.S. Pat. 6,468,278 B1 and German Patent DE 43 41 980 A1, for example.
However, one disadvantage of the monoaxially stable plate-screw joint is that the osteosynthesis plate must be aligned very accurately on the bone because of the fixed plate-screw angle. Even minor misalignment of the plate can cause misalignment of the screws that cannot be corrected.
Finally, various embodiments of polyaxially stable plate-screw joints are known from U.S. Pat. No. 5,053,036, French Patent 2,790,198, U.S. Pat. No. 5,954,722, U.S. Pat. No. 5,520,690 and U.S. Pat. No. 5,607,426. The bone screws here are screwed into the bone through bores in the osteosynthesis plate at an angle that can be selected within certain limits of freedom. Since the screws do not naturally have any angular stability at first, they are secured at the previously set angle by means of lockable split taper sockets, which are between the screw and the bore in the plate. The outside surface of the split taper socket and/or the outside surface of the screw head has a spherical shape to ensure free angular mobility.
An alternative approach to free angular mobility without the need for split taper sockets is described by Wolter, Schumann, Seide (1999): The Universal Titanium Fixator; Trauma Berufskrankheiten [Trauma, Occupational Diseases] 1, 307-19. A thread is created in the bore by a tool without creating chips and a bone screw is then screwed through this bore. However, the final direction of the screw is defined without any play when the thread is created.
With the known implants, a fundamental distinction must be made between those that can exert tensile forces on the bone and those that are merely screwed into the bone. With many indications, it is important for the fracture position to secure the bone fragment by tightening the bone screw against the osteosynthesis plate in a targeted manner to thus achieve an anatomically correct position of the respective bone fragments predetermined by the shape of the plate. Of the embodiments cited, this is possible only with the implants according to U.S. Pat. No. 5,520,690 and the very similar U.S. Pat. No. 5,607,426. However, the implants proposed there have a relatively great structural height in the area of the screw head due to the design, so that the use spectrum is limited, in particular in cases of unfavorable soft tissue coverage.
For fixation of bone screws in any directions, European Patent 0 201 024 A1 discloses an arrangement in which a first plate is in contact with the bone and receives a bone screw polyaxially in a receiving bore, whereby the receiving bore is dimensioned in relation to the head of the bone screw so that the head projects above the first plate. After tightening the bone screw, a cover plate is to be placed on this projecting head and tightened by screwing it against the first plate. Due to the pressure of the head of the bone screw through the cover plate, this creates a frictional engagement between the head of the bone screw and the first plate, causing a directionally rigid connection according to the statements in this document. Here again, however, a relatively great structural height is unavoidable due to the design. Furthermore, this design is relatively complicated and is therefore expensive to manufacture.
The object of the present invention is to design an implant—without restricting the freedom in the angle of the bone screws—i.e., without restricting the conventional use of a particular type of implant—so that the bone screws can be secured particularly reliably and nevertheless easily with the smallest possible plate thickness and the smallest possible total structural height in the area of the bone screws. An example of a conventional use of a type of implant should also include in particular the possibility of being able to exert tensile forces on the bone when the bone screw is screwed into the bone so that the osteosynthesis plate can be tightened against the bone and thus a secure fixation between the bone and the osteosynthesis plate can be achieved.
This object is achieved by an osteosynthesis plate or a comparable orthopedic implant with a receiving bore and a clamping bore, whereby the receiving bore is oriented for holding a bone screw directly or indirectly, and the clamping bore runs essentially perpendicular to the osteosynthesis plate or parallel to the receiving bore. A recess is arranged in the area of the aforementioned bores, preferably between the bores, in relation to the receiving bore and to the clamping bore such that widening of the clamping bore causes a constriction of the receiving bore. Directly holding the bone screw is understood to mean that no split taper socket is arranged between the bone screw and the receiving bore, but instead the two are in direct contact. When it is held indirectly, however, the bone screw is inserted into a clamping screw, which in turn sits in the receiving bore.